The road to bis‐periazulene (cyclohepta[def]fluorene): Realizing one of the long‐standing dreams in nonalternant hydrocarbons

Abstract

The chemistry of nonalternant hydrocarbons has recently experienced a considerable resurgence. As the relationship between naphthalene and azulene shows, an important design concept for a nonalternant framework is to replace the two hexagons of an alternant system with a pentagon and heptagon pair.

Potential singlet fission chromophores of azulene-containing tetracene analogues: a theoretical study

“…41 Moreover, bisperiazulene has an open-shell triplet ground state. 42 These results confirms the effectiveness of inducing the diradical character with the azulene unit. However, little has been reported about their SF properties despite the excellent performances of azulene-containing polyaromatic hydrocarbons.…”

Structural Expansion of Cyclohepta[def]fluorene towards Azulene‐Embedded Non‐Benzenoid Nanographenes

“…[18] However, the synthesis of cyclohepta[def]fluorene (b) and its derivatives is quite challenging owing to their intrinsic instability. [19] Very recently, the solution synthesis of the triaryl derivatives of cyclohepta[def]fluorene (c) and the benzo-extended cyclohepta[def]fluorene derivatives (such as diaryl dibenzocyclohepta[def]fluorene, d) was independently reported by Yasuda's group [12] and our group. [13] It was found that the introduction of substituents onto 4,8,10-positions and dibenzoextension onto 1,9,10-positions of cyclohepta[def]fluorene not only increases the molecular stability but also changes the electronic structure (Figure 1a).…”

“…As a representative example, pyrene ( a ) can be transformed into one of its non‐alternant isomers, that is, cyclohepta[ def ]fluorene ( b ) (namely bis‐periazulene), by replacing the inner naphthalene unit with azulene (Figure 1a), which is predicted to be an open‐shell Kekulé structure with a low triplet ground state [18] . However, the synthesis of cyclohepta[ def ]fluorene ( b ) and its derivatives is quite challenging owing to their intrinsic instability [19] . Very recently, the solution synthesis of the triaryl derivatives of cyclohepta[ def ]fluorene ( c ) and the benzo‐extended cyclohepta[ def ]fluorene derivatives (such as diaryl dibenzo‐cyclohepta[ def ]fluorene, d ) was independently reported by Yasuda's group [12] and our group [13] .…”

Constructing 1-Ethoxyphenanthro[9,10-e]acephenanthrylene for the Synthesis of a Polyaromatic Hydrocarbon Containing a Formal Azulene Unit

“…For example, the formation of an azulene unit of singlet diradicaloid 2 , a peri -dibenzo-fused derivative of benzo­[ f ]­azulene (azulene fragment is shown in blue color, Figure a), was achieved at the late stage using a synthetic protocol that was applicable to construct two additional azulene-embedded diradicaloids with an extremely small singlet–triplet energy gap. Compound 2 is a benzo-extended cyclohepta­[ def ]­fluorene (benzo-CHF), which was long theoretically predicted to possess a singlet–triplet bistable ground state; though lately it was established as a singlet diradicaloid in the ground state . Notably, in Feng’s report, additional two benzannulation (shown in bold hexagons, Figure b) of benzo-CHF 2 afforded Dibenzo-2 with a singlet diradicaloid ground state.…”